Collector mirror for plasma-based, short-wavelength radiation sources

Abstract

The invention is directed to a collector mirror for short-wavelength radiation based on a plasma. It is the object of the invention to find a novel possibility for managing the temperature of a collector mirror for focusing short-wavelength radiation generated from a plasma which allows an efficient thermal connection to be produced between the optically active mirror surface and a thermostat system without the disadvantages relating to space requirements or high-precision manufacture of the collector mirror. This object is met, according to the invention, in that the collector mirror has a solid, rotationally symmetric substrate which comprises a material with high thermal conductivity of more than 50 W / mK and in which channels for cooling and temperature management are incorporated in the substrate so that a heat transport medium can flow through directly and for rapidly stabilizing the temperature of the optically active mirror surface. Heat of transient temperature spikes which occur in pulsed operation for plasma generation at the mirror surface and which temporarily exceed the temperature average by a multiple is quickly dissipated.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application claims priority of German Application No. 10 2005 017 262.8, filed Apr. 12, 2005, the complete disclosure of which is hereby incorporated by reference.BACKGROUND OF THE INVENTION[0002]a) Field of the Invention[0003]The invention is directed to a collector mirror for short-wavelength radiation sources based on a plasma having an optically active mirror surface with high reflectivity for the desired short-wavelength radiation and in which means for cooling the mirror body are provided due to the position of the collector mirror in the immediate vicinity of the plasma which has an extremely high temperature. The invention is preferably applied in radiation sources for large-scale production in semiconductor lithography which emit in the extreme ultraviolet spectral region (EUV region).[0004]b) Description of the Related Art[0005]Plasma-based radiation sources for semiconductor lithography (primarily EUV sources) are essential...

AI Technical Summary

Benefits of technology

[0009]It is the primary object of the invention to find a novel possibility for managing the temperature of a collector mirror for focusing short-wavelength radiation generated from a plasma which allows a high-performance thermal connection to be produced between the optically active mirror surface and a thermostat system without the disadvantages relating to space requirements or requirements for the high-precision manufacture of the collector mirror. Further, processes for the additional or subsequent surface coating of the collector mirror are also possible without greater expenditure.

[0010]According to the invention, this object is met in a collector mirror for short-wavelength radiation sources based on a plasma which has an optically active mirror surface with high reflectivity for the desired short-wavelength radiation and in which means are provided for managing the temperature of the mirror body because of the position of the collector mirror in the immediate vicinity of the hot plasma, wherein the collector mirror has a solid, rotationally symmetric substrate which comprises a material with high thermal conductivity of more than 100 W / mK, and in that flow channels for cooling and temperature management are incorporated directly in the substrate so that a heat transport medium can flow through directly for rapidly stabilizing the temperature of the optically active mirror surface at a defined level, wherein heat from transient temperature spikes which occur in pulsed operation for plasma generation and which temporarily exceed the temperature average prevailing at the mirror surface by a multiple is quickly dissipated.

[0012]The channels for the heat transport medium are preferably in the form of cylindrical bore holes, and the connections for the supply of heat transport medium can advantageously be screwed into threaded bore holes in the substrate in a simple manner.

[0014]Further, it has proven advantageous when the substrate is constructed in two parts. The first part, as substrate, has the optically active mirror surface and the finely structured channels incorporated on the back, and the second part, as cover, is designed to cover the channels so as to be tight against media.

[0017]Another possibility for fastening the cover is a material bond with the substrate which can advantageously be achieved by means of a solder connection or by cementing or gluing.

[0024]With the solution according to the invention it is possible to realize a reliable cooling and temperature management of a collector mirror for focusing short-wavelength radiation generated from a plasma which allows an efficient thermal connection to be made between the optically active mirror surface and a thermostat system without having to tolerate disadvantages with respect to space requirement or requirements for high-precision manufacture of the collector mirror. Further, processes for the additional or subsequent surface coating of the collector mirror are also made possible without increasing expenditure.

Problems solved by technology

While glass is suitable in principle for producing collector mirrors for plasma-based radiation sources because it can be produced with excellent surface quality, its thermal conductivity (≈1 W / mK) is too small to use it as a substrate of a collector mirror in an EUV source for semiconductor lithography.

In particular for alternating-layer mirrors with direct (non-grazing) radiation reflection, dissipation of this radiation load and thermal load constitutes a technical challenge because the temperature average of the alternating-layer mirror may not exceed several tens of degrees Centigrade.

A degradation in the alternating-layer system takes place even when the mirror is heated (only temporarily) to above approximately 200° C. and leads to a substantial reduction in reflectivity.

At the outputs of an EUV source for large-scale production in semiconductor lithography at which several tens of kW excitation output are converted, cooling of this kind is not sufficient for ensuring a long lifetime of the reflection coating of the collector mirror.

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